deletions | additions       diff --git a/From_lab_to_field_Although__.md b/From_lab_to_field_Although__.md  index f26654e..bc5943f 100644  --- a/From_lab_to_field_Although__.md  +++ b/From_lab_to_field_Although__.md 
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Modelling is a useful tool to test the scope of mycorrhizal weathering concepts.   In this section we will review the available field data and modeling work on mycorrhizal weathering in this research field.  Without doubtdoes  vegetation have has  a substantial positive effect on soil mineral weathering \citep{Berner_1992}. It remains the question though, how much influence the associated mycorrhizal fungi have on this vegetation effect.  Due to the slow kinetics of soil mineral weathering, and the complex soil matrix, a direct estimation of the contribution of mycorrhizal fungi on the weathering process is challenging.  Three different approaches have been adopted to address the impact of mycorrhizal weathering: 1) historical weathering markers, 2) stable isotopes to trace the source of tree nutrients and 3) quantifying incubated minerals in contrasting soils. 
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Tunnels, as described in \citet{Jongmans_1997} are the only quantifiable fungal markers of weathering that remain visible over geological time.  Unfortunately, fungal tunneling either reflects only a small portion of the total effect of fungi on the weathering process, or the fungal impact is negligible, as tunneling contributes less than 0.5% tot total mineral weathering \citep{Smits_2005}.  In a recent paper \citet{Koele_2014} showed that mineral tunneling is not exclusively found under ectomycorrhizal vegetation, but also in forest soils, never exposed to ectomycorrhizal vegetation.   This suggests that tunnels can be formed by other means (e.g. arbuscular mycorrhizal or saptrophic fungi). Work by Quirk and others indeed show that also arbuscular mycorrhizal fungi create weathering trenches on mineral surfaces \citep{Quirk_2012,Quirk_2014} . \citep{Quirk_2012,Quirk_2014}.  ##Isotope tracers  Stable isotopes of especially Ca and Sr have been used extensively to source the origin of Ca in drainage water .   Applied to plant tissues, it could potentially trace plant nutrients back to their primary source.   It has been primarily mostly  used to studythe  apatite weathering. Apatite is a calcium-phosphate mineral, and as P has no stable isotopes, the uptake dynamics can only be studied via the Ca ion (or potentially the ¹⁸O/¹⁶O in the phosphate group) . group).  In most rocks and soils apatite is the sole primary P source. But Ca Its its  contribution to the soil solution Ca pool is minor compared to other minerals. If the Ca isotope ratio in the plant is more similar to the signature in apatite than to the signature  in the soil solution, it indicates that the plant takes up Ca directly from the apatite crystal. As the apatite crystals crystal sizes  are below the root scale, it indicates a selective uptake via mycorrhizal hyphae colonizing apatite grains. In an influential paper \citet{Blum_2002} applied this technique, but as technique to a temperate mixed forest. But because  in their study area, area  the different mineral sources did have similar Ca isotope ratios, they used the ratio between Ca and Sr instead. Using element ratios, instead Uptake and soil cycling processes are likely to fractionate more between different elements than between different isotopes  ofisotope ratios, increases  the risk of fractionation. same element.  Already in 1926 \citet{Fay_strontium_1926} warned for the use of Ca/Sr ratio to trace sources of Ca. Most of the Ca taken up by trees comes from litter recycling. In a comparablenortheastern  mixed forest, also in northeastern USA,  the annual Ca import from weathering in the rooting zone is less than 0.3% of the annual Ca uptake , which was a 4 times smaller flux than the annual atmospheric deposition \citep{Dijkstra_2002}. A closer look at the data presented in \citet{Blum_2002} clearly separates ectomycorrhizal trees with a high Ca/Cr ratio (the two coniferous evergreen  species) and ectomycorrhizal trees with a low Ca/Sr ratio in their leaves (the two deciduous species). Although in principle this difference could be explained by host specific mycorrhizal communities, with the both coniferous evergreen  species hosting mycorrhizal fungi with stronger capability to weather apatite, a more obvious explanation is that Ca/Sr fractionation is different during throughfall and litter recycling between these coniferous evergreen  and deciduous trees. Up to now, isotope techniques have not provided convincing evidence of a major mycorrhizal contribution in mineral weathering.  ##Mineral incubations 
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Based on this model, and its application in many boreal forest soils, \citet{Sverdrup_2009} concludes that protons are the major weathering agent in these soils, organic chelators like oxalate only play a minor role.  The main critique on the PROFILE model from a fungal point of view is that simulating weathering on a soil layer scale overlooks the potential importance of local, fungal-scale, high concentrations of fungal weathering agents\citep{Finlay_2009}.  But, as illustrated in \citet{Smits_2009}, due to the specific dissolution kinetics of the main dissolution reactions, local concentration of weathering agents do not automatically lead to higher dissolution rates.   Following from the kinetics, the PROFILE modelling approach only underestimates the action of certain weathering agents if they are exuded to and stay within a micrometer range of specific mineral surfaces, or are specifically exudated near active surface sites. sites \citep{Smits_2009}. On the other hand, if fungal action takes place specifically at surface sites with weaker crystal binds (e.g. crystal steps, fresh cracks), the fungal impact on overall dissolution rate will be higher.  In contrast to the PROFILE model, the weathering module developed by \citet{Taylor_2011} incorporates exudation of fungal weathering agents on the mineral grain scale.   The model explicitly assumes that in systems with ectomycorrhizal trees all tree-soil interactions (uptake and exudation) takes place via the ectomycorrhizal fungi acting close only to nutrient-bearing minerals (so not close to quartz which is the most common mineral in top soils).